In the related art, due to the ability to detect temperature in a non-contact manner, infrared sensors have been used widely, for example, in an automatic lighting system that detects the presence of a person and performs lighting control based on the movement of the person and for detecting the temperature inside a microwave and detecting the temperature distribution in food being cooked. The infrared sensor is a device that outputs a voltage corresponding to the amount of infrared light received in an internal sensor chip.
Although the detection field of view of the infrared sensor is determined by specifications, in practice, unnecessary infrared light from outside the field of view also enters into the device. When the infrared light enters into the sensor chip by internal reflection in the package, the infrared light becomes thermal noise. Thus, there is a problem in that detection accuracy deteriorates.
As an example of such an infrared sensor, one in which a number of resistors (mono devices) are arranged on a surface in a matrix form, a change in resistance value resulting from an increase in temperature occurring when infrared light is projected to the surface is extracted as a signal, and an image signal is created and output based on the signal is known, for example. In such an infrared sensor, it is necessary to supply a predetermined bias current in order to extract the change in resistance as a signal, and the temperature of the infrared sensor may increase due to the Joule heat resulting from the bias current. Thus, another signal is generated from the infrared sensor due to the increase in temperature caused by the Joule heat, and this signal is output as noise in the infrared sensor.
In order to prevent such noise caused by the bias current, a structure in which, as shown in FIG. 8, a ring-shaped inner 250 is fixed in a case 220 so that even when disturbance noise enters into the case 220, the disturbance noise is radiated without reaching a sensor chip 230 mounted on a circuit substrate 210, and only infrared energy collected by a lens is guided to the sensor chip 230 is proposed (Patent Document 1).
Moreover, Patent Document 2 discloses a structure in which, as shown in FIG. 9, a circuit substrate 110, an electron cooler 140, and an infrared detecting device 130 are sequentially mounted on a package 120 so that a shielding trunk 150 is provided so as to cover the infrared detecting device 130, and the shielding trunk 150 is cooled by the electron cooler 140. By cooling the infrared detecting device 130 with the electron cooler 140, an increase in the temperature of the infrared detecting device 130 caused by a bias current is prevented. By cooling the shielding trunk 150, an increase in the temperature of the infrared detecting device 130 caused by heat radiating through the shielding trunk 150 is prevented, and noise is reduced. Moreover, by forming a light incident window 150a open to the shielding trunk 150 into an appropriate opening size, incident infrared light will not leak outside the respective mono devices of the infrared detecting device 130, and the occurrence of noise is prevented.
Furthermore, an infrared sensor module in which as shown in FIG. 10, rather than shielding a thermopile device (infrared sensor device) 330, by covering the thermopile device 330 with an inner cap 350 thermally connected to a stem 310, the followability of the temperature of a cold junction of the thermopile device 330 with respect to a variation of the temperature of the stem 310 is improved (Patent Document 3). In this configuration, the thermopile device and a thermistor are covered by the inner cap so that the inner environment is thermally connected to the stem, and secondary radiation from the case 320 or the inner cap 350 does not reach the thermopile device 330.
In recent years, a mounting structure in which IC chips such as ASICs (Application Specific Integrated Circuits) for signal amplification processing are incorporated has been proposed. With this configuration, it is possible to reduce drawing of wirings, and in theory, to achieve further improvement of detection accuracy. However, when IC chips are incorporated into a housing in such a way, it is necessary to use a metal housing as a countermeasure to electrical noise. In this case, in particular, since the optical reflectivity of metal is high, internal reflection is likely to occur. Thus, a structure in which thermal noise is more likely to occur is realized. Moreover, in some cases, the extent of influence of thermal noise on IC chips for signal amplification processing becomes non-negligibly high.